1. Field of the Invention
The present invention generally relates to a method and apparatus for nondestructive testing, monitoring of technological processes, determining structural integrity, noise and vibration control, and mine detection. More specifically, the present invention relates to a phase-amplitude modulated electromagnetic wave (PAM-EW) vibrometer.
2. Related Art
Existing remote vibrometers are generally based on coherent laser generated signals. These devices, known as laser-doppler vibrometers, require precision and expensive optical elements (acousto-optic modulators, gas lasers, mirrors, beam splitters, etc.) A very precise, very coherent source is required, i.e. very stable phase characteristics. Fine adjustments are necessary to achieve a desirable effect. As a result, the laser-doppler vibrometers are quite expensive and delicate instruments are best suited for laboratory use.
Another serious drawback of the conventional remote sensing devices is their high sensitivity to unwanted vibration of the transmitting/receiving assembly (TRA). In fact, vibrometers measure only relative velocity/displacement between the vibrating object and the TRA. Since the sensitivity of the conventional laser-doppler vibrometers is very high it is very difficult to isolate the TRA from such small vibrations especially under field conditions. In addition to this, conventional vibrometers are susceptible to so-called cosine error. That is, if the incident electromagnetic wave is not precisely perpendicular to the irradiated surface, an error proportional to the cosine of the angle between the line of radiation and a normal to the surface is introduced.
Efforts of others in this area include U.S. Pat. No. 5,883,715, to Steinlechner, et al., entitled Laser Vibrometer for Vibration Measurements; U.S. Pat. No. 5,897,494, to Flock, et al., entitled Vibrometer; U.S. Pat. No. 5,495,767, to Wang, et al., entitled Laser Vibrometer; and U.S. Pat. No. 4,768,381, to Sugimoto, entitled Optical Vibrometer.
None of these efforts of others teaches or suggests all of the elements of the present invention, nor do they disclose all of the advantages of the present invention.